The medically and economically important African trypanosomes, such as Trypanosoma brucei, elicit protective, trypanocidal responses, which, because of antigenic variation, are only temporarily effective. Similar responses also occur against T. lewisi, one of the nonpathogenic rodent trypanosomes, but the benign nature of this parasite actually results from a unique reproduction-inhibiting (ablastic) immune response by the host. Furthermore, the eliciting antigens occur on the surface of the parasites and in the blood as circulating exoantigens. Antigens (ablastinogens) that elicit the ablastic response to T. lewisi have now been isolated, and a search for counterparts in the African trypanosomes is now feasible. Therefore, the following studies are proposed: 1.) Ablastinogens have been separated from trypanosome extracts on immunoadsorbent columns made with ablastic IgG and will be similarly isolated from surface coat material, pellicular membranes and blood; antigens from all these sources will be characterized by standard biochemical techniques, compared and further purified to determine the minimum number required for a response. 2.) Immune complexes have been isolated from the blood and shown to contain ablastinogen. Similar studies will be made to determine whether the variant antigens of T. lewisi also occur as immune complexes. 3.) The variant antigens of the two antigenic variants of T. lewisi will be isolated on immunoadsorbent columns, characterized and compared with ablastinogen. 4.) Monoclonal antibodies will be made to T. lewisi. These antibodies will be used in immunoadsorbent columns to isolate ablastinogen and the variant antigens, for functional studies of ablastic activity, and to determine whether purer biologically active antigens can be obtained. 5.) A culture system that allows continuous growth of bloodstream forms of T. lewisi has been found and will be developed further for use in long-term studies of ablastic effects, and to study exoantigen release and immune complex formation. 6.) The cell surface of antigenic variants of T. brucei will be analyzed for shared, accessible, non-variant antigens that are not ordinarily immunogenic, since there is evidence that such antigens occur. Among these may be counterparts of ablastinogen that could be used to circumvent the obstacles to practical immunization created by antigenic variation.